The present invention generally relates to devices and methods for sealing well bores and bore holes, and more specifically to a method and apparatus which effectuates the sealing of well bores and bore holes through the use of mineral substances formed into compressed plates, where the compressed plates have the property of swelling into an expanded state upon contact with, and absorption of, a liquid. Among other applications, the disclosed method and apparatus may be used to seal off well bores and bore holes associated with the production, injection and/or disposal of water, hydrocarbons, and other fluids. The disclosed method and apparatus may also be utilized in the sealing of bore holes associated with the storage and/or disposal of nuclear materials. The terms well bore and bore hole are hereinafter used interchangeably and are intended to refer to any excavation which is deeper than the measurement across its opening.
Sealing or plugging a well bore may be required for a number of reasons, with each situation presenting different operational constraints. One common application of sealing or plugging a well bore occurs when a well is to be abandoned. Proper well abandonment is required to protect freshwater aquifers from fluid migration from other zones penetrated by the well bore. For example, in oil and gas wells, proper abandonment is necessary to prevent migration of fluids from a hydrocarbon zone to an adjacent freshwater zone. Proper abandonment is necessary to protect the freshwater aquifer and to confine hydrocarbons. Agencies which regulate oil and gas production typically require a cement plug, or plugs of other substances with adequate physical properties, to be placed across specified intervals for abandonment purposes. According to the usual regulatory requirements, the plug must extend from the total depth of the well or from at least one hundred feet below the bottom of each oil and gas zone to at least one hundred feet above the top of each oil or gas zone. In a cased well, all perforations are to be plugged and the plug is to extend at least 100 feet above the top of a landed liner, the uppermost perforations, the casing cementing point, the water shut-off holes, or the oil or gas zone, whichever is highest. An additional plug is required across fresh-saltwater interfaces. Fluids having the proper weight and consistency to prevent movement of other fluids into the well bore are placed across all intervals not otherwise plugged.
The most common method of sealing or plugging a well bore is pumping in a cement slurry to the zone of interest. However, depending upon the situation, design of an appropriate cement slurry can be complicated, requiring a number of different and expensive additives. Moreover, in the usual situation, a number of expensive resources must be assembled for the operation. Equipment such as a drilling rig, work-over unit, portable mast, pumping and blending works, bulk trucks, etc., and the personnel to operate the equipment, may be required to stand by during the course of the operation. The equipment and personnel may continue to stand by even after application of the cement slurry has concluded, waiting for the cement to harden to an acceptable strength.
Another known disadvantage of cement is that it can degrade over time, such that it might contract or crack, thereby providing channels through which fluid migration might take place. Even without such degradation, cement is known to have some permeability, potentially allowing fluid flow through a cement plug.
As an alternative to using cement as a plugging or sealing material, it is known to use high quality course ground chemically unaltered sodium bentonite (hereinafter referred to as sodium bentonite). Sodium bentonite is the name given to clay containing over 90% sodium montmorillonite, one of several minerals within the smectite group. These substances exhibit pronounced swelling tendencies when exposed to a hydrating liquid. Sodium bentonite has long been identified as a material with excellent plugging ability. Dry sodium bentonite will swell to a size ten to twenty-five times its dry state when hydrated. Hydrated sodium bentonite has extremely low permeability, a much-desired characteristic for an isolation plug. It has long been identified as a suitable material for plugging monitoring wells, seismic shot holes, mining shafts, exploratory holes, water wells, and oil and gas wells. Sodium bentonite has also been identified as an excellent plugging material for high-level nuclear waste repositories because of its ability to confine waste migration and resist alteration over time.
A simple method is known for using sodium bentonite to create a plug in a well bore. This method comprises simply pouring a small granular form of dry sodium bentonite into the well bore from the surface, filling the hole from the bottom upward. However, sodium bentonite in granular form (having a diameter of no greater than xe2x85x9c inches) will swell quickly upon encountering water or other liquids. As sodium bentonite hydrates and expands, its density decreases, resulting in a reduced falling velocity and increased exposure to well bore liquids before reaching the desired depth. This characteristic of the granular form of sodium bentonite creates a risk that the material will bridge and plug off the well before a plug is achieved at the desired depth. Therefore, the use of finely ground sodium bentonite is generally limited to shallow well bores up to 100 feet in depth. This material might also be used in a deeper well bore containing very little liquid.
Because of the risk of bridging off, alternative placement methods for sodium bentonite have been proposed. In one method, bentonite is poured from the surface while rotating an auger drill within the well to force the bentonite downward into the well. In another method, bentonite chips are encapsulated and poured into the well from the surface. It is also known to use chemically unaltered compressed sodium bentonite nodules, which are poured from the surface and, ideally, fall to the intended plugging zone where they accumulate until the desired plug height is reached. The nodules are typically manufactured from roll type briquetting and compacting machines. Compression of the nodule reduces the hydration and expansion rates typical of uncompressed sodium bentonite, allowing use of this material in deeper applications. However, all of these methods continue to rely upon surface pouring the sodium bentonite into the well. Notwithstanding improvements offered by these methods, there remains a potential for the material to bridge off above the desired depth of the plug, particularly where there is an excessive pour rate. Well bore irregularities, obstructions, extreme directional changes, fluid interfaces, viscous emulsions, and other common well bore occurrences may cause the initially poured grains, capsules or nodules to decelerate. Subsequently poured sodium bentonite may catch up with the initial material, causing the grains, capsules or nodules to congregate, and ultimately to bridge.
Nodules may present a further disadvantage. Depending upon how the nodules are packaged and delivered to the well site, sodium bentonite residue may accumulate from nodule-to-nodule contact. If this residue is included in the surface pour, the residue will begin to hydrate almost immediately upon contact with well liquids, thereby increasing the liquid viscosity and decreasing the velocity of the surrounding nodules. It is also possible for nodule-to-nodule contact to take place during the surface pour or as the nodules are falling into place within the well bore, which once again can generate sodium bentonite residue resulting in an increase in the liquid viscosity.
Another disadvantage of surface pouring sodium bentonite grains, capsules or nodules is that forming a plug by surface pouring these materials is limited to generally vertical wells. These materials will tend to collect or bridge in highly deviated and/or horizontal well bores, rendering it unlikely that the lateral portion of the well bore can be effectively plugged.
The present invention is directed to an apparatus and method for sealing well bores and bore holes which meets the needs identified above. The apparatus comprises a first plate having a top surface, a bottom surface, and a peripheral edge surface bound by the top surface and the bottom surface. The peripheral edge surface has a maximum width defined by the linear distance between a first point on the peripheral edge surface to an opposite second point on the peripheral edge surface. The plate has a first compressed state wherein the maximum width is smaller than the diameter of the well bore or the bore hole to be sealed. The plate is formed from a mineral substance. This mineral substance has the property of swelling into a second expanded state upon contact with a liquid and absorption of that liquid. The apparatus requires means for placing the plate within the well bore or bore hole. Among the mineral substances which may be utilized for forming the plate are members of the group of clay minerals known as smectites, including sodium montmorillonite, calcium montmorillonite, sodium bentonite and calcium bentonite.
In one embodiment the plate may be round. For this embodiment, the plate has a first compressed state wherein the plate has a diameter smaller than the diameter of the well bore.
A method of sealing well bores and bore holes is also disclosed. The method comprises the steps of first disposing a mineral substance into a mold, where the substance has the property of swelling upon contact with a liquid and absorption of the liquid. The mineral substance is then molded into a plate, where the plate has a top surface, a bottom surface, and a peripheral edge surface bound by the top surface and the bottom surface. The peripheral edge surface has a maximum width defined by the linear distance between a first point on the peripheral edge surface to an opposite second point on the peripheral edge surface. The plate is compressed into a first compressed state where the maximum width is smaller than the diameter of the well bore. The plate is thereafter placed into the well bore with placement means. The plate is thereafter exposed to liquid such that the mineral substance swells into a second expanded state upon contact with the liquid and absorption of the liquid.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings.